Heat exchangers with fins attached to surfaces for heat transfer enhancement are widespread in many industrial and household applications. The most commonly encountered constructions include round tubes with helically wound aluminum or steel fins, in which the fins are maintained in thermal contact with the tube by the tension imparted during the winding process. Several variations exist, including sometimes a groove formed in the tube in which the fin is embedded for better thermal contact, and in other instances the fin being high frequency resistance welded to the tube for increased strength. While these tubes are relatively easy to make, they present a number of disadvantages, among which are high pressure loss in the fluid flowing through the fins and inefficient use of fin material. The high pressure loss is caused by the resistance to flow presented by the numerous tubes around which the fluid must turn, as indicated by arrows in FIG. 1. The inefficient use of the fins is caused by the stagnant fluid regions forming behind each tube in which heat transfer does not occur; the portions of the fins adjacent to these regions, represented by hashed areas in FIG. 1, could as well be cut out without reducing the total heat transfer, but of course this cannot be done conveniently.
The round finned tube disadvantages can be eliminated by the flat finned tube construction. As shown in FIG. 2 the flat finned tube is made of an oblong cross section tube with two parallel flat walls and straight fins attached to the two walls. A flat finned tube can replace several round tubes, and its slender profile, allows fluid flow with no turns, and thus with no resistance, as indicated by arrows in FIG. 2. There are no stagnant fluid regions and the fins are heat transfer effective over their entire extent. The net result is a heat exchanger with improved energy efficiency and of markedly reduced size. Consequently, the flat finned tube has :first found applications where these two factors are at a premium, namely in automotive vehicular heat exchangers, such as car radiators, and in the air-cooled steam condensers of electric power generation plants.
Due to the large surface carrying capacity of the flat finned tubes, a heat exchanger consists often of a single row of tubes as shown in the perspective sketch of FIG. 3. As shown in the figure, the fins can be attached to each of two neighboring tubes, uniting all tubes in a structure of high rigidity which is further advantageous and not possible to achieve with round finned tubes. In typical applications a liquid or a condensing fluid is circulated through the inside of the tubes, and cooling air is forced over the external tube surfaces and through the fins. Not shown in the figure are the supporting frame, the headers for bringing the inside fluid in and out of the heat exchanger, and the fan and ducts for air circulation, considering that these are easily understood conventional components.
In actual applications the flat tube dimensions can vary greatly. Thus, in a vehicular heat exchanger, the tubes may have approximately dimension B of 20 millimeters, dimension D of 2 millimeters, fin width H of 10 millimeters, fin thickness of 0.2 millimeters, fin pitch of 2 millimeters, and the tube length of about 0.5 meters; in a power plant air-cooled steam condenser the tubes may have approximately dimension B of 300 millimeters, dimension D of 20 millimeters, fin width H of 40 millimeters, fin thickness of 0.4 millimeters, fin pitch of 3 millimeters, and the tube length of about 10 meters. This wide range of sizes available for the flat finned tubes is not possible with the round finned tubes and constitutes an additional advantage allowing greater versatility and further reduction of the heat exchanger dimensions.
Despite their advantages over the round tubes, the flat finned tubes, as currently practiced, present a number of deficiencies which have prevented their widespread use in place of the round tubes.
One difficult problem is that of creating a fin to tube attachment of high strength. While on a round tube the fin is maintained rather securely around the tube by the tension imparted during its winding, in a flat finned tube the fin is simply posed on the flat tube wall and must be attached by a bonding process. As shown in FIG. 4, which is an enlargement of the encircled area of FIG. 3, the fins are usually supplied as waved strips attached to the tube at the wave apexes. The attachment area is of limited width, of approximately twice the fin thickness which is very small compared with the fin height, thus creating a spot for the amplification of stresses due to tube movements by vibration or thermal expansion. Thus, unless the attachment is of considerable strength everywhere, the fins can become detached losing thermal contact with consequent loss of the heat exchanger performance. U.S. Pat. No. 3,693,710 to Dorsnin (1972) attempts to solve this problem by bonding with metal solder and providing a large number of perforation in the fin wave apex through which the solder penetrates to the other side of the fin, thus increasing the area of attachment; this method has the disadvantage of increasing the extent of the fin to solder junction which is exposed to the environment and thus aggravates the galvanic corrosion problems inherent in soldered junctions.
The strength problem is addressed by U.S. Pat. No. 4,949,543 to Cottone et al. (1990), by creating a metallurgical bond between each fin and the tube, through brazing in a controlled atmosphere furnace. While the bond strength is therefore increased, the process is carried at high temperature, exceeding 600.degree. C., and when applied to aluminum fins the high temperature leads to full annealing of the fins which are thus softened and made prone to denting during assembly or cleaning, causing closure of the air channels and subsequent reduction of the heat exchanger performance. The process is rather expensive and is not universally applicable to all desirable tube and fin materials.
In the European patent EP 0 490 210 A1 to Borchert et al. (1992), the solution to the strength problem is given by spot welding the fins to the tube with a laser beam from the underside of the tube wall; subsequently the tubes are immersed in a bath of molten zinc to provide the needed thermal contact between each fin and the tube, but not to add to the strength. The tubes have to be made of two halves which are subsequently welded. The process seems highly laborious and costly, with the many steps involved, and with the large bath of molten zinc required for tubes of about 10 meters length. Moreover, the process is limited to application of steel fins to steel tubes, and cannot be used for other materials, such as aluminum fins which are often preferable for their high thermal conductivity and increased heat transfer performance. The fin style used is essentially similar to that depicted in FIG. 4; in addition a different fin style is also indicated, consisting in separately mounted fins, as shown in FIG. 5, which while not better from the strength point of view would be prohibitively laborious to make in many applications, such as the car radiator with its small and very numerous fins.
Another difficult problem with the flat finned tubes is that of assuring the corrosion resistance of the fin to tube junction which is narrow and long. As seen from FIGS. 4 and 5, representative for the current state of the art, each fin is attached to the tube by a very narrow junction formed of some bonding filler material, shown in the figures by the darkened spots at the base of the fin. The total length of these junctions over the many fins and tubes of a heat exchanger is very large, e.g., of about 300 meters for a vehicular heat exchanger, and about 300,000 kilometers for a power plant air-cooled steam condenser. The tube and fin materials are often dissimilar, such as steel and aluminum, or if they are of the same metal the solder or brazing filler is still a different metal, such that galvanic corrosion is likely to occur since the junction is exposed to the environment formed by the fluid flowing over the fins. Even if this fluid is ambient air, it often contains moisture such as from road splash and rain, laden with salts, or even acids in an industrial environment. The junction must be of very specialized nature to resist corrosion and a different solution must be given for every environment and every desirable combination of fin and tube materials. The quality control methods should be quite extraordinary and costly to assure the required quality of the very long junction line, and in practice defects are always allowed with the result that part of the junction line will suffer corrosion and become detached, with consequent reduction of the heat exchanger performance. Solutions given to this problem to the present time are only partial, costly, and not devoid of associated problems.
The corrosion problem is solved by U.S. Pat. Nos. 4,949,543 (1990), 5,042,574 (1991), 5,102,032 (1992), and 5,277,358 (1994), all to Cottone et al., for aluminum fins attached to a steel tube by brazing in a nitrogen atmosphere furnace. The tube is made of steel coated with an aluminum layer, and the fins are formed from multilayered laminated aluminum sheet with two layers of brazing aluminum alloy. The rather complex manufacturing process takes place at high temperature and therefore must employ fluxing and zone temperature control to prevent formation of brittle and corrosion prone aluminum-iron alloy in the fin to tube junctions. The use of aluminum coated steel in place of simply steel, and of multilayered laminated aluminum sheet add to the cost and the method cannot be extended to other desirable tube and fin materials.
In the European patent EP 0 490 210 A1, already mentioned, the solution to the corrosion problem is given by coating with zinc large heat exchanger sections after assembly, by immersion in a bath of molten zinc. Since that patent refers to power plant steam condensers the sizes involved are very large and the required molten zinc bath is correspondingly large and costly. It can also be mentioned that zinc is not an entirely environmentally friendly metal and its spread in the environment should be prevented, which is very difficult with the very large amounts of zinc handled, of very high temperature and evaporating from the bath. Moreover, most of the zinc applied in a power plant steam condenser will wash into the ground during the life of the equipment, and may reach the groundwater. This is not a negligible problem with the large amounts of zinc involved which can be as much as 3000 tons for one power plant.
It may be apparent from what was said above that the flat finned tube construction and methods of manufacturing should be improved to reduce cost and expand the range of applications.
In car radiators and power plant steam condensers the operating temperature is rather low, of less than about 100.degree. C., and the same is true for other potential applications of the flat finned tube, such as condensers and evaporators in air conditioning units. By contrast, the current manufacturing processes are based on brazing and zinc aided bonding which are complex high temperature processes.
For low temperature applications, such as power plant steam condensers and car radiators, it would be desirable to attach the fins to the flat tube by bonding with an organic structural adhesive,--a highly developed technique, with many adhesive formulations available for temperature levels in excess of those required. If bonding with an organic structural adhesive could be adopted to flat finned tubes this would represent a great improvement of the manufacturing process which could be carried at ambient temperature and by simple means. Desirable though as it is, bonding by organic structural adhesives has not been applied to date to flat finned tubes for several reasons. First, with reference to FIG. 4 again, it can be noticed that the fin to tube junction is very narrow, of the order of one fin thickness for each fin; if the bonding filler material, indicated in the figure by the darkened spots at the base of the fin, is an organic adhesive, this would not have sufficiently high thermal conductivity to allow the transfer of heat between tube and fins through the narrow cross section available. Second, the bond would not have sufficient strength to withstand the stresses transmitted to the joint by tube vibration and thermal expansion movement. Third, the very long line of the joint discussed before will expose the thin layer of adhesive directly to the moisture carrying environment leading to loss of strength in relatively short time. Although organic structural adhesives are being applied to achieve structures of high strength even in the highly demanding aerospace industry, their application is adequate only for joints of sufficiently large contact area, and line contacts such as formed here by the fin to tube junction are to be avoided. Even if the fin style is modified as shown in FIG. 5, the contact area is still insufficient to achieve strength, and the problem of the very long line of exposed junction is still present. Thus, this highly desirable manufacturing method could not be adopted for flat finned tubes.
Besides the application domains already mentioned there exist others in which the flat finned tube would be beneficial. Such is the potential application to boiler tubes in heat recovery steam generators for combined cycle power plants, in which round finned tubes are presently used with the combustion gases circulated through the fins. For these the current state of the art discussed above would certainly not work due to the corrosive nature of the combustion gases and the high temperatures involved. Neither would the current state of the art be applicable to other high temperature heat exchangers such as gas turbine recuperators.
The present invention resolves the above identified problems of the flat finned tubes and makes possible their application at low temperatures as well as at high temperatures, for cooling with air as in car radiators and power plant steam condensers, or for heating with combustion gases in power generation equipment. Moreover, the present invention makes possible a simpler and more economic manufacturing method.
In addition, the invention will be also applicable to plate-fin heat exchangers in which the flat tubes are replaced by flat parallel plates and the fins are applied on both sides of each plate, as depicted schematically in FIG. 6. Plate-fin heat exchangers are found in numerous applications, such as in air liquefaction and gas turbine compressor intercoolers. The tube walls or the plates can be thought of as representing partition walls separating two fluids exchanging heat, while having in common the same problems associated with the attachment of a straight fin to a flat wall. Therefore the invention is described in the general terms of finned partition walls rather than just flat finned tubes.